Estimation of Lifetime of Plastic Gears


This paper gives an overview on the state of art in plastic gear resistance calculation. The main problem with plastics is the dependency of the stress cycle curve (Woehler line) with temperature. Today, more plastic gears (as in automobile headlights) are used in a high temperature range. Furthermore, flank resistance depends strongly on lubrication (lifetime may vary by a factor of ten and more, if oil, grease lubricated or dry running).

As no secure data for plastic gears is available, how can nevertheless plastic gear design and life time prediction be improved? The best strategy is to use the feedback of existing reducers. Plastic gearboxes, before starting production in big series, are normally submitted to endurance tests. If these tests are used to check also the real lifetime limits - or by increasing test length, or by increasing applied torque - these results can be used to define the required safety factors for future gear design. This procedure has been very successful, and will be described with some examples.
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